The present disclosure relates generally to uninterruptible power supplies (UPSs), and particularly to control systems for economic control thereof.
UPSs are employed in a variety of applications where a constant source of power is desired at a load. A typical UPS system involves an inverter feed path, also generally referred to as the inverter, that is operably connectable in parallel with a bypass feed path, also generally referred to as the mains. The mains may be connected to a utility, but may also receive power from some other supply not connected to a utility electrical grid. The inverter may receive power from the same source as the mains, but may also receive power from some other supply.
There are several types of UPSs depending on their operation mode. Double conversion UPSs offer the maximal protection level as the load is always fed by the inverter. This type of UPS, however, may have limited efficiency, which results in higher operation cost, particularly for large units. On the other hand, with line-interactive UPSs, the load is fed by the mains and the inverter is used to correct the shape of the load voltage. This type of UPS has a better efficiency, but may not be able to maintain the quality of the output voltage when the mains frequency is variable. To some extent, it is possible to combine the advantages of the two types of UPSs with the concept of ECOMODE. The basic idea is to feed the load by the mains while constantly monitoring the mains quality. As soon as a problem on the mains is detected, the load is switched on to the inverter. The concept of ECOMODE is presently used on UPSs without an output isolation transformer, however, such an arrangement suffers from high efficiency losses.
Accordingly, there is a need in the art for a control strategy allowing for the use of an ECOMODE UPS system that overcomes the aforementioned drawbacks.